Recently, the development of the backbone network communication line as mainly focusing on the optical fiber communication technology is proceeding steadily and, in such a situation, the electronic wirings in the information terminal are becoming a bottleneck. Against such background, instead of the conventional electric circuit substrate in which all signal transmissions are made by using the electric signal, the optoelectronic composite substrate (optical waveguide device) of the type that transmits high-speed parts by the light has been proposed, in order to compensate the limit of transmission speed of the electric signal.
In the optoelectronic composite substrate, the light signal is transmitted by the optical waveguide including such a structure in which the core layer is surrounded by the cladding layers.
In an example of the optical waveguide manufacturing method, first, the lower cladding layer, the core layers, and the upper cladding layer are formed sequentially on the substrate. Then, the resultant structure is processed by a rotary blade so as to divide the core layer from the upper cladding layer, and thus the groove portion having the inclined surface inclined at an angle of 45° to the light propagation direction is formed. Then, the metal layer having light reflection property is partially formed on the groove in the core layer, and then the metal layer on the inclined surface is used as the light path conversion mirror.
Then, a light that propagates through the core layer is reflected in the vertical direction by the light path conversion mirror on the inclined surface of the groove, thus the light path is converted.
A related art is disclosed in Japanese Laid-open Patent Publication No. 2007-293244.
As explained in the column of the preliminary matter described later, there is the case in either the case where two optical devices (the light emitting element and the light receiving element) are optically coupled to both end sides of the optical waveguide by two rows respectively, or the case where the optical devices in which the light emitting portions or the light receiving portions with two rows are built in the inner part are optically coupled to them respectively. In such optical waveguide, the different light paths with two routes are prepared according to the optical devices, and the light path conversion mirrors are arranged in the different positions in every light path.
In case the optical waveguides serving as the different light paths are stacked and formed, a distance of the light path in the optical device which is optically coupled to the optical waveguide located on the lower side is set longer by a thickness of the optical waveguide located on the upper side. Therefore, an increase of an optical coupling loss is caused, and as a result enough reliability of the optical characteristics cannot be obtained.